The card will almost certainly come with an adaptor.
When comparing a 9800pro VS 9600XT there is really no contest...
http://www.guru3d.com/article/content/117/8/
Lots of benchmarks to be seen there and all show more or less the same thing; 9800pro >= 9600XT.
As for the made my ATI vs 3rd part manufacturers there is really no difference. The cards are always essentially the same but often have varying cooling solutions. Sometimes 3rd party manufacturers provide a better HSF but its always good to read a review first and make sure that its not worse than the default one.
Some companies such as Gainward provide additonal functionality in their cards, for example they marketed a watercooled 5900U a while ago.
Their cards commonly have higher clocked memory/core but at a slightly higher price point.
Hybrid Mode
